Compressed Air Rules of Thumb
By CasUpdated Aug 2026 It's always handy to know some rules of thumb. They allow you to quickly estimate the size of a compressor, or the energy that it will cost to run it, for example.
Here they all are in one cheat sheet, in imperial and SI units. Below the cheat sheet you'll find the stories behind them: where the numbers come from, and when they stop working.
The cheat sheet
Sizing and output
| Rule of thumb | In SI units |
|---|---|
| Every compressor horsepower delivers 4 to 5 CFM, at 100 psi | Every kW delivers 2.5 to 3.2 l/s |
| Air motors need about 30 CFM at 90 psi, per horsepower | About 19 l/s per kW |
| It takes 7 to 8 hp of electricity to produce 1 hp of air power | The same ratio in any units |
| A tool fed 100 psi when it only needs 50 psi uses about 75% more air | The same at 7 bar vs 3.5 bar |
Water
| Rule of thumb | In SI units |
|---|---|
| 1 gallon of water per day, per compressor horsepower (worst case) | 5 liters per day per kW |
| Saturated air at 100 °F carries about 2 gallons of water per hour, per 100 CFM | About 7.5 l/h per 50 l/s |
| Every 20 °F drop in saturated compressed air condenses half of the water vapor | Every 11 °C drop |
| Water-cooled aftercoolers need about 3 GPM of cooling water per 100 CFM | About 11 l/min per 50 l/s |
Energy and money
| Rule of thumb | In SI units |
|---|---|
| Electricity cost per year: about $500 per HP (6,000 hours per year, $0.10 per kWh) | About $600 per kW per year |
| 1 hp running 24/7 for a year costs about $750 | 1 kW running 24/7: about $900 per year |
| 2 psi of pressure drop costs 1% extra energy | 0.1 bar costs 0.7% extra energy |
| Lowering the working pressure by 15 psi saves about 8% energy | 1 bar lower saves about 8% |
| Every 7 °F hotter intake air costs 1% more energy | Every 4 °C hotter intake air |
| A dirty inlet filter (10 inches of water gauge lost) costs 2% output | 25 mbar lost at the inlet costs 2% |
| Compressed air costs about $0.25 to $0.42 per 1,000 cubic feet of free air | About $9 to $15 per 1,000 m³ |
| Three 1/8 inch leaks lose about 78 CFM; one 1/4 inch leak, about 100 CFM | A 3 mm hole leaks about 11 l/s at 7 bar |
| A 50 hp compressor rejects about 126,000 BTU per hour as heat | Nearly every kW you put in comes out as heat: 37 kW in, about 37 kW of heat out |
Storage
| Rule of thumb | In SI units |
|---|---|
| Air receivers: at least 4 gallons per CFM of compressor capacity | 30 liters per l/s, or 500 liters per m³/min |
Temperatures and oil
| Rule of thumb | In SI units |
|---|---|
| Discharge temperatures before the aftercooler: oil-injected screw 175 °F, oil-free screw 350 °F, single-stage piston 350 °F, two-stage piston 250 °F | 80 °C, 180 °C, 180 °C, 120 °C |
| Every 18 °F above 200 °F halves the compressor oil life | Every 10 °C above 95 °C |
The stories behind them
4-5 CFM per horsepower
For every horsepower, a compressor delivers 4 to 5 cfm, at 100 psi pressure. In other words: a 1 horsepower compressor will output around 4 to 5 cfm at 100 psi, and a 10 HP unit will output around 40 to 50 cfm.
If we need a higher pressure, say 125 psi, this rule of thumb doesn't work anymore. We will need more HP to compress to 125 psi with the same output. In other words: per HP, the compressor output will be smaller at 125 psi.
In SI units: for every 1 kW of power, a compressor delivers 2.5 to 3.2 l/s.
1 gallon of water per day, per horsepower
For every horsepower, a compressor generates 1 gallon of water per day (worst case). A compressor makes compressed air, but it also produces lots of water! The water comes from the ambient intake air. How much water is generated depends mainly on the relative humidity and temperature of the ambient air.
In this rule of thumb, a day is 24 running hours, and this is really worst-case, like running your compressor in a tropical rain forest.
Not all of that water condenses into liquid. Typically, in a system without an air dryer, 1/4 will condense into liquid in the after cooler, 2/4 will condense in the piping system (if given time to cool down) and the last 1/4 will stay in the compressed air as water vapor.
In SI units: 5 liters per day, per kW of compressor power.
A more typical example. Ambient temperature 20 degrees Celsius, relative humidity 50%, a 45 HP compressor running 24 hours a day. A 45 HP compressor takes in around 7 m³ per minute. At 20 degrees and 50% humidity, a quick calculation gives us that this compressor takes in around 23 gallons (85 liters) of water per day.
Air receivers: 4 gallons per CFM
Actually, I would like to say 'the bigger the better'. A bigger air receiver is ALWAYS better than a smaller one.
A big air receiver helps to keep the compressed air system stable. It acts like a buffer for sudden air consumption spikes. It keeps the pressure more stable: with a bigger air receiver, it requires more air flow out of the receiver for the same amount of pressure drop. This makes the compressors start and stop less, which is great for their health. It also helps in removing oil and water from the compressed air (condensation in the air receiver).
But let's use this rule of thumb as a bare minimum, ok :)
Converted to SI units: air receivers should be 30 liters for every 1 l/s of compressor capacity. Or for bigger systems: 500 liters for every 1 m³/min.
Electricity cost: about $500 per HP per year
This rule of thumb is for a compressor that runs around 6,000 hours per year (2 full shifts per day).
For example, let's take a 45 HP / 33 kW air compressor. If we run it for 6,000 hours per year, at 10 cents per kWh, we pay: 33 × 6,000 × 0.10 = $19,800 in electricity cost per year! Close enough for a rule of thumb :)
Of course, this heavily depends on the compressor size, the cost of electricity, and of course the running hours.
2 psi pressure drop costs 1% extra energy
Pressure drop, every compressed air system's worst enemy! Pressure drop is created by resistance in the system between the compressor and the air consumer. It means we have less pressure at the consumer. So we must create a higher pressure at the compressor, to account for the pressure drop. This costs us extra energy, and therefore money.
As a rule of thumb: 2 psi of pressure drop costs 1% extra energy. In SI units: every 0.1 bar of pressure drop costs 0.7% extra energy. Not as catchy, I know :)
Typical discharge temperatures
Not really a rule of thumb, but here are some typical discharge air temperatures of industrial air compressors:
- Oil-injected rotary screw: 175 °F or 80 °C
- Oil-free rotary screw: 350 °F or 180 °C
- Single-stage piston compressor: 350 °F or 180 °C
- Two-stage piston compressor: 250 °F or 120 °C
Of course, temperatures will vary depending on running conditions like cleanliness of coolers, ambient temperatures, loading times, etc.
Oil degradation at high temperature
Oil in rotary screw compressors will degrade quicker when the compressor runs at high temperatures (read: change your oil more often!). As a rule of thumb: for every 18 °F above 200 °F, the compressor oil life is reduced by 50%. In SI units: for every 10 °C above 95 °C.
In other words: if your screw compressor runs at or above 220 °F / 105 °C, you should change your oil at half of the normal running hours!